1. Field of the Invention
The present invention pertains to air traffic surveillance, notably civilian air traffic surveillance and, more particularly, to air-ground cooperating systems that enable the aircraft present in a certain volume to be located in terms of radial distance and azimuth, and to be interrogated.
2. Description of the Prior Art
These cooperating systems include a surveillance radar, called a secondary surveillance radar, which uses a directional and rotating antenna to transmit amplitude modulated and phase modulated pulses at the 1030 MHZ frequency and transponders on board aircraft which reply by means of pulse trains that are amplitude modulated at the 1090 MHz frequency. For civil aviation, interrogations and replies may be given in different modes: at present in the A and C modes and in future in the S mode which are distinguished notably by the formats of the interrogation and reply pulse trains.
In the S mode, which enables a selective interrogation of the aircraft through the use of an identification number proper to each aircraft, and resolves the problems of traffic congestion encountered with the A and C modes, the response consists of a preamble formed by a sequence of four pulses, the positions of which are invariable, followed by a data block composed of 56 or 112 pulses of 0.5 .mu.s coded in modulation of position on successive time intervals of 1 .mu.s. A redundant coding with parity check at the data block enables error-free decoding of a message localized by the detection of the preamble pulses when it has undergone deterioration in a zone of less than 24 .mu.s.
It turns out to be the case that the deterioration of the message often goes beyond 24 .mu.s, notably in the case of multiple paths where the deterioration may affect the entire message, and in cases of interlacing of a response in S mode with several responses in standard A or C mode.
Multiple-path phenomena frequently occur in terminal zones when the aircraft is close to the ground and to the secondary interrogation radar. They are due to reflections, of the reply from the transponder, off buildings and off the ground. These reflections arrive with time lags in relation to the direct reply and disturb it.
The present invention is designed to improve the detection of the messages contained in these responses in the presence of radioelectrical disturbances.